> Hello Tom, > > I had a conversation with Sam S. from TSC the > other day, and he said that it's probably not > possible to get <10ns GPS accuracy anyways > due to the multipath issues, Ionospheric issues, > antenna survey issues, thermal issues etc.
Yes, with a standard GPS receiver or GPSDO, I very much agree with this. See also the links to GPS papers I posted earlier today which will give you a feel for what level of accuracy or stability you get from various GPS time transfer techniques. Note accuracy and stability are two different goals. Related to that, GPS-based frequency reference products are plentiful, cheap, and in widespread use while GPS-based time transfer products are few, very expensive, and have a small use base. I think most of us time-nuts use GPS as a source of precise time interval (GPS as a ~ 1e-13 stable frequency reference) rather than a source of absolute time (~ ns accurate UTC). This is why uncalibrated GPS receivers work for all of us. See Rick's papers on M12 calibration at USNO. http://www.gpstime.com/ http://tycho.usno.navy.mil/ptti/ptti2002/paper9.pdf Note NIST's use of regular GPS receivers (Oncore?) for frequency _stability_ (not time _accuracy_): http://www.tf.nist.gov/service/fms.htm So 10 ns _stability_ with GPS over a day is quite doable on the cheap. < 10 ns _accuracy_ with GPS is quite another matter and requires a huge amount of work. On this list, DougH is probably the only one who's pulled it off: http://www.leapsecond.com/time-nuts.htm > Our units typically average the GPS 1PPS over > 30 minutes, so having less than 3.33ns error on > the 1PPS capture may not improve things much > because while the error stays always at +-3.33ns > it get's averaged over the measurement intervall. 3.3ns / 30 m = 2e-12 and sets your lower bound. But it would depend on your choice of LO and PLL if this is a limiting factor or not. Here's something to try: deliberately degrade your 1PPS TIC resolution in software and see what effect it makes on the stability of your RF or 1PPS output. /tvb _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
